This study will compare the current standard BiST mode of ventilation with the new iVAPS mode. The main study hypothesis is that the iVAPS mode, initiated in a single daytime trial, will result in a reduction of the number of respiratory therapist interventions and changes in ventilator settings as compared with the standard BiST mode. This will be assessed over a period of one year. In addition this study will test whether the iVAPS mode is superior to BiST mode with respect to: comfort and ease of use; improvement in nighttime and daytime symptoms of hypoventilation; compliance (hours used per day); physiologic parameters (daytime and overnight oxygen saturation and transcutaneous CO2 level).